This study evaluated the variations in VOC concentrations before and after the COVID-19 pandemic in 2019 and 2021, respectively, in the Seoul Metro underground subway station lines 1–8. The total VOC concentrations in 253 underground stations ranged 1.5–566.0 µg/m3, with a mean of 43.8 µg/m3, in 2019 and 16.7–392.2 µg/m3, with a mean of 99.9 µg/m3, in 2021. Notably, the mean total VOC concentrations on the underground station platforms for both years were lower than the recommended limit of 400 μg/m315. However, the health effects associated with poor indoor environments are most likely driven by chronic low-level exposure to some of these compounds, including gaseous chemicals with a high vapor pressure at room temperature(i.e., VOCs)16.
The VOC concentrations in all eight metro lines before the COVID-19 pandemic were lower than those after the pandemic (Table 2). During the pandemic, the average VOC concentration in Seoul Metro underground stations (lines 1–8) increased 2.28 times compared to that before COVID-19. The result was the opposite in underground subway stations with PM, which decreased compared to that before the COVID-19 pandemic11. Telecommuting was considered the main reason that PM concentration decreased during the COVID-19 pandemic. This policy considerably reduced the number of passengers using the subway during the outbreak compared to before the outbreak.
The increase in the VOC concentration during the COVID-19 pandemic was attributed to the increased frequency of disinfection. The frequency of the disinfection cycle was increased for all platform structures within the reach of passengers, such as escalator handrails and elevator buttons, to prevent the spread of COVID-19. The Seoul Metro increased the quarantine disinfection frequency to at least twice a week for platforms and at least four times a day for surfaces frequently in physical contact. Frequent quarantine disinfection seems to be a factor that increases VOC concentrations. The use of disinfectants, such as air fresheners and multipurpose surface cleaners, focuses on viral elimination and not necessarily on emissions, with the assumption that more use is better17. However, exposure to these cleaning products has been associated with adverse effects on human health. Cleaning products and disinfectants are complex chemical mixtures that often contain multiple respiratory sensitizers and irritants18. Nazaroff and Weschler19 provided evidence for a link between adverse health outcomes and chemical exposure from cleaning products. Recent nationally-representative population-based studies conducted across the United Kingdom and Sweden found that 32.2% of the general population reported health problems when exposed to all-purpose cleaners and disinfectants20,21. Common disinfectants recommended for use against COVID-19 include quaternary ammonium compounds (QAC), hydrogen peroxide, and bleach (sodium hypochlorite). Comprehensive exposure assessment reports for cleaning agents and chemical disinfectants reveal that they include alcohols (ethanol, 928 ± 958 µg/m3; and 2-propanol, 47.9 ± 52.2 µg/m3); ketones (acetone, 22.6 ± 20.6 µg/m3); peroxygen compounds (hydrogen peroxide, < 11.0–511.4 parts per billion (ppb) and peracetic acid, < 2.2–48.0 ppb); monoethanolamines (0.005–0.559 mg/m3); ethylene glycol mono-n-butyl ether (49.479 to 58.723 mg/m3); benzyl alcohol (0.864–5.446 mg/m3); and QAC (benzyldimethyldodecyl ammonium chloride, 0.23 μg/m3 and benzyldimethyltetradecyl, ammonium chloride 1.5 μg/m3)22,23,24,25. Frequent disinfection with these substances can affect cleaners who directly handle them; therefore, an overall status and health impact survey of cleaners is needed. Although focusing on different indoor environments, former studies observed that many VOCs are present with disinfectant use during the COVID-19 pandemic26.
We confirmed that the depth of the underground platform influences the levels of VOCs, and a greater difference in the levels of VOCs was observed after a depth of 25 m (Fig. 2). Insufficient air exchange rate and improper management of ventilation cause the high VOC levels; therefore, robust indoor air circulation and ventilation enhance the IAQ level and minimize the health risk in subway platforms. The Seoul Metro installed high-performance air purifiers on the platforms to increase the efficiency of ventilation for the underground environment during the COVID-19 pandemic period, thereby providing clean air and a pleasant indoor environment for the platform users. A recent study verified the efficiency of air purifiers in reducing PM concentrations27.
The Seoul Metro has been installing highly efficient air purifiers, monitoring IAQ, and frequently cleaning underground stations in an effort to reduce airborne pollution since 2007. However, the VOC concentration was higher during the COVID-19 pandemic than before. The present study found that the deeper the underground station platform, the greater the difference in the VOC concentration between 2019 and 2021 (Fig. 3). This might be attributed to the high frequency of quarantine disinfection, which was conducted at least twice a week for the platform and at least four times a day for escalator handrails and elevator buttons. The Seoul Metro should pay more attention to the ventilation of underground stations for IAQ management and reducing VOC concentration, focusing on platforms at depths n > 25 m, if the use of quarantine disinfectants is to be continued. Alternatively, the frequency of quarantine disinfection using agents that emit VOC pollutants may need to be reduced. A recent study28 proposed the use of mitigation strategies, such as air curtains at subway exits, magnetic filters at the top of the ventilation opening, and adsorbing materials that can adsorb harmful gases, to improve the IAQ in underground subway stations. These methods can also be used to reduce the VOC concentrations.
The average VOC concentration increased as the number of passengers decreased during the COVID-19 pandemic (Table 4). Humans are a potent mobile source of VOCs in indoor environments. Several hundred VOCs are emitted into the surrounding air via exhalation and dermal emissions29. Oxidants present in indoor air (e.g., ozone or hydroxyl radicals) can produce VOCs by coming in contact with a human body surface (hair and skin) and clothing30,31. However, we found there was no significant association between the number of passengers and the VOC concentration, indicating that quarantine disinfection is more likely the main source of VOCs in this study.
Although this is the first study to identify the factors influencing VOC concentrations in Seoul Metro subway stations before and during the COVID-19 pandemic, there are a few limitations. First, the measurements of VOCs in lines 1–8 may not necessarily reflect an association with public health outcomes. Our understanding of the effects of VOCs on human health is limited because of analytical difficulties in measuring real ambient air concentrations and evaluating personal exposure, as well as poor knowledge regarding the toxicity of multiple compounds32. Second, other environmental factors, such as ventilation status, air changes per hour, filtering system, temperature, humidity, building materials, and air purifier performance, were not investigated; these factors can affect VOC concentration in underground subway stations. Third, it was impossible to accurately determine the exact air pollutants in the samples because the VOC composition could not be analyzed. Singh14 reported that VOCs concentration gradually decreased benzene by − 50% and − 15% for toluene during the lockdown period compared to before the COVID-19 pandemic in 2019, respectively. Then, an increase in the total VOC concentration by 16% was observed in post-pandemic periods; this may be due to the re-opening of commercial places, various industries, and transportation14. Finally, the VOC concentration for each station cannot be considered wholly representative because the VOC was measured once for 6 h. Despite these limitations, this study was conducted at 253 underground subway stations with 506 samples in 2019 and 2021 on lines 1–8 at a large scale, which are difficult to access using standard air sampling methods. Our results provide useful indicators for reducing the VOC concentration by reducing the use of disinfectants at depths of over 25 m in underground subway stations. The quantitative concentrations of VOCs are still an efficient indicator for understanding how VOCs can be reduced by applying various systematic methods, such as installing air purification in underground stations, using fewer disinfectants, and increasing the ventilation frequency.